Chronic myocardial ischemia is the leading cardiac illness affecting the general population in the Western world. Since the occurrence of angina symptoms or objective physiological manifestations of myocardial ischemia signifies a mismatch between myocardial oxygen demand and the available coronary blood flow, the goal of therapy is to restore this balance. This can be achieved either by attempting to prevent further disease progression through modification of risk factors, or by more aggressive modes of therapy such as reducing the myocardial oxygen demand (i.e. reducing the heart rate, myocardial contractility or blood pressure) by using anti-anginal medications, or by restoring the blood supply by means of mechanical interventions such as percutaneous transluminal angioplasty or its variants, or coronary artery bypass surgery, coronary angioplasty (PTCA) or bypass surgery (CABG). When CABG is selected as the treatment option, its success may be limited by the inability to provide complete revascularization in those patients in whom the artery that supplies a viable but underperfused myocardial territory is not graftable because of diffuse disease, calcifications, or small size. Complete revascularization cannot be achieved in up to 37% of patients undergoing CABG. This number is probably much lower today. However, patients who undergo complete revascularization have improved 5-year survival and angina-free survival compared with patients who have incomplete revascularization. Therefore, an adjunctive treatment strategy is warranted in patients undergoing CABG if complete revascularization is not possible. Percutaneous catheter-based revascularization is often precluded secondary to the same attributes that made the myocardial territory ungraftable: diffuse disease and small or calcified vessels.
The field of angiogenesis research was initiated 30 years ago by a hypothesis that tumors are angiogenesis-dependent. Folkman, J. “Tumor angiogenesis: therapeutic implications.” N. Engl. J. Med. 285: 1182–1186 (1971). Shortly thereafter, in the early 1970's, it became possible to passage vascular endothelial cells in vitro for the first time. Bioassays for angiogenesis were developed subsequently through that decade. The early 1980's saw the purification of the first angiogenic factors. Clinical applications of angiogenesis research are being pursued along three general lines: 1) prognostic markers in cancer patients; 2) anti-angiogenic therapy (in cancer treatment); and 3) angiogenic therapy (treatment of heart disease).
In discussing the field of angiogenesis, it is important to differentiate 3 different processes that contribute to the growth of new vessels. Vasculogenesis is the primary process responsible for the growth of new vasculature during embryonic development, and it may play an as yet undefined role in mature adult tissues. Arteriogenesis refers to the appearance of new arteries possessing fully developed tunica media, while true angiogenesis describes the growth of collateral-like vessels lacking the development of media. In the case of coronary circulation, arteriogenesis is usually taken to mean new, angiographically visible epicardial vessels while angiogenesis refers to thin-walled intramyocardial collaterals.
Occlusion of coronary arteries is often associated with development of collateral circulation in patients with atherosclerosis. Although the existence of collateral circulation in such patients is associated with improved clinical outcomes, the net effect is rarely adequate to compensate fully for the flow lost to occlusion of native epicardial coronary arteries. A number of growth factors have been associated with myocardial and peripheral limb ischemia, particularly basic fibroblast growth factor (bFGF), acidic fibroblast growth factor (FGF-1), and vascular endothelial growth factor (VEGF), which have been shown to induce functionally significant angiogenesis in animal models of myocardial and limb ischemia. These promising preclinical results have rapidly lead to the study of these growth factors in patients with chronic myocardial ischemia using intracoronary (IC), intravenous (i.v.), and local delivery (myocardial injection).
Therapeutic myocardial angiogenesis is a novel approach to the treatment of myocardial ischemia based on the use of proangiogenic growth factors to induce the growth and development of new blood vessels to supply the myocardium at risk. Angiogenesis is a complex process involving endothelial and smooth muscle cell proliferation and migration, formation of new capillaries, and extracellular matrix turnover. Various heparin-binding growth factors, including basic fibroblast growth factor (FGF-2), acidic fibroblast growth factor, and vascular endothelial growth factor (VEGF) induce angiogenesis in chronic myocardial ischemia. Given the typically long time course of new collateral vessel development, most attempts to stimulate myocardial angiogenesis have used methods of prolonged growth factor delivery, including gene therapy, continuous infusions, repeated injections, or sustained release polymers. However, some of these options are not feasible or practical in patients with ischemic heart disease, making single dose administration, if effective, a potentially superior strategy in these patients.
Angiogenesis is a complex process that involves endothelial cell migration and proliferation, extracellular matrix breakdown, attraction of pericytes and macrophages, smooth muscle cell proliferation and migration, formation and “sealing” of new vascular structures, and deposition of new matrix. A number of growth factors, including the fibroblast growth factors (FGF) and vascular endothelial growth factors (VEGF) are integrally involved in the angiogenic response in ischemic conditions and in certain pathological states. The availability of these factors has led to studies, which have demonstrated a therapeutic benefit in various animal models of acute and chronic myocardial ischemia. In particular, basic fibroblast growth factor is an attractive candidate as an agent for therapeutic angiogenesis.
The therapeutic goal of attempting to ameliorate chronic ischemic conditions through revascularization by administration of various protein growth factors is feasible only due to the chronic nature of the condition and the resulting long-term time scale for treatment. In acute clinical situations, such as myocardial infarct, or therapeutic procedures likely to lead to reperfusion injury, the luxury of long time scales for revascularation is not available. However, the administration, via various routes, of growth factors such as FGF has been demonstrated to be effective in reducing the effects of myocardial infarct within a time frame that precludes a therapeutic contribution from the angiogenic function of these proteins. See, for example, my earlier U.S. Pat. No. 4,296,100, the disclosure of which is hereby incorporated specifically by reference. Thus, by a mechanism yet to be elucidated, protein growth factors such as FGF and VEGF and related proteins are capable of demonstrating a therapeutic utility in situations involving acute damage to the heart.